1. Field of the Invention
The invention relates to an apparatus and a method to be employed in semiconductor device fabrication for depositing a film on a substrate by chemical vapor deposition, and more particularly to an apparatus and a method for accomplishing chemical vapor deposition in which solid or liquid source is used.
2. Description of the Related Art
There has been employed chemical vapor deposition (hereinafter, referred to simply as "CVD") in order to form a capacity film or an insulating film of a semiconductor device. In CVD, a source including an ingredient or ingredients of which a film is intended to be made of is introduced in a gas condition into a reaction chamber in which a substrate is placed, and a gas source is made into reaction by heating to thereby deposit a film on a substrate. Thus, a source to be used for CVD is necessary to be gaseous at room temperature (RT).
However, as materials such as tantalum oxide, strontium titanate and lead titanate zirconate have been employed for semiconductor device fabrication, source which is in solid or liquid condition at room temperature is often employed for CVD. The reason is that gas sources containing ingredients of those materials are scarce.
An amount of source which is gaseous at room temperature might be readily controlled by employing a flow rate controller such as a mass flow controller (hereinafter, referred to simply as "MFC"). In contrast, source which is in solid or liquid condition at room temperature has in general a low vapor pressure, and hence it would be more difficult to control an amount of solid or liquid source than an amount of a gas source.
When CVD is accomplished employing a solid or liquid source, there has been conventionally carried out a method in which a container containing a solid or liquid source is heated to thereby evaporate the solid or liquid source, and the thus evaporated source is introduced into MFC for directly controlling a flow rate of the evaporated source. Since this method controls an amount of source to be supplied in the form of a flow rate, the method provides sufficient repeatability.
However, the above mentioned conventional method has a problem that a system for supplying source, including MFC, has to be heated at a temperature higher than a condensation point of a source. Accordingly, a temperature of a source has to be kept lower than a maximum temperature against which MFC has heat resistance, because MFC has in general the most poor heat resistance.
In addition, there has to be created a difference in pressure between a gas inlet port and a gas exhaust port of MFC in order to control a flow rate of a source by means of MFC.
Accordingly, it is impossible in the above mentioned conventional method to supply a source having a low vapor pressure and hence generating no pressure at a temperature lower than the maximum temperature against which MFC has heat resistance, even if the gas exhaust port of MFC which is in communication with a CVD reaction chamber is made evacuated. That is, the method has a problem that only materials having a relatively high vapor pressure can be employed as a source.
In another method of accomplishing CVD employing a solid or liquid source, a container containing a source therein is kept at a constant temperature, and a predetermined amount of carrier gas controlled in a flow rate by MFC is introduced into the container, to thereby introduce the source into a CVD reaction chamber together with the carrier gas. In general, an inert gas such as argon (Ar) is used as a carrier gas.
In this method, a system for supplying a source to a CVD reaction chamber, including the container, is necessary to be heated at a temperature higher than a condensation point of a source, either. However, since MFC is located upstream of the source container, a degree in a temperature increase is not so high as that of the previously mentioned method. Hence, this method makes it possible to employ a source which has a low vapor pressure and is difficult to be evaporated. This method has been widely employed because of this advantage. In this method, an amount of a source to be supplied to a CVD reaction chamber is controlled by a temperature of a source container, a pressure in a source container, and a flow rate of a carrier gas.
In a method in which a source is supplied together with a carrier gas, an amount of a source is defined in accordance with the following equation (A). EQU n.varies.F.times..alpha.Ps/(P-.alpha.Ps) (A)
In the equation (A), n indicates an amount of a source to be supplied, F indicates an amount of a carrier gas, P indicates a total pressure in a source container, Ps indicates a saturation vapor pressure of a source, and .alpha. indicates a degree of saturation of a source. The degree of saturation .alpha. is a coefficient indicating whether a partial pressure of a source in a source container reaches a saturation pressure thereof. A source is supplied to a CVD reaction chamber in such a condition that the degree of saturation .alpha. is equal to 1 (one), that is, a partial pressure of a source in a source container is equal to a saturation pressure thereof. The saturation pressure Ps of a source is dependent only on a temperature, and is higher at a higher temperature. As would be obvious in view of the equation (A), when the degree of saturation .alpha. is equal to 1, an amount of a source to be supplied to a CVD reaction chamber is dependent on a temperature of a source, a pressure in a source container, and a flow rate of a carrier gas. These factors are optimized in film deposition by CVD in order to obtain a desired film deposition rate and film composition.
However, the above mentioned method employing a carrier gas for supplying a source to a CVD reaction chamber has a problem that it is impossible to know an amount of a source having been actually supplied to a CVD reaction chamber. Because, even if an amount of a carrier gas, a temperature of a source container, and a pressure of a source container are fixed in optimal values to thereby deposit a film, an amount of a source having been actually supplied to a CVD reaction chamber is varied due to fluctuation in a temperature of a source container and decrease in an amount of a source caused by repeated film deposition.
As mentioned above, it is impossible in the conventional method to keep an amount of a source at a constant, target amount, since the conventional method does not have means for detecting fluctuation in an amount of a source. The fluctuation in an amount of a source causes fluctuation in a film deposition rate which in turn causes deviation in a film thickness, and also causes deviation in film composition from an intended composition in the case of polyphyletic film formation, resulting in deviation in film characteristics from intended ones. Namely, there cannot be obtained sufficient repeatability in film deposition by CVD, which in turn reduces productivity in semiconductor device fabrication.